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RE: Structural Slab on Grade

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We typically design our PT slabs with pour strips which are sections of the
slab that are not cast until at least 28 days (longer is better) after the
adjacent slab has been cast to allow for some of the shrinkage to occur
without tearing the slab apart.  The pour strips are usually 3'-9" wide to
allow the use of single sheets of ply (not a problem at grade).  The joints
usually have shear keys and quite a lot of additional mild reinforcing to
prevent cracking at or near the joints.  This method could be extended to a
foundation slab quite readily.  We try to keep the width:length ratio to a
maximum of 1:1.5 with 1:1 being preferred.

The trick will be to provide a sub-base that provides as little restraint to
shortening as possible.  With the pile foundation that is going to be near
impossible.  

There is an article in this month's (Jan 2001) ACI International about the
construction of super flat crack "free" slabs for warehouse use.  We have
never been able to convince an owner that these admixtures are cost
effective, but I read, year after year about how they have improved and
miracle success stories, so given a pinch of salt and good design practices
they probably do work. I can fax you the article if you don't get ACI
International.  Send me your fax number offline to the address below.

Good luck

Nicholas Blackburn
mailto:nblackburn(--nospam--at)fdgoak.com
fax: (510) 553-2163


-----Original Message-----
From: Hicklin, Ray [mailto:rhicklin(--nospam--at)DLRGROUP.com]
Sent: Monday, January 22, 2001 12:38 PM
To: 'seaint(--nospam--at)seaint.org'
Subject: Structural Slab on Grade


I am designing a concrete slab for a large exposed area, 270' x 630'. Due to
the poor soil conditions, the slab will be designed as a structural slab on
grade supported by piling and pile caps. The slab will be designed for live
loads up to 350 psf. The question is what to do about crack control. If this
was truly a slab on grade we would provide saw cut joints, but don't really
want that in a structural slab. If it was an elevated slab the relatively
small stiffness of the columns would provide limited restraint to resist the
shrinkage forces. Post-tensioning and shrinkage compensating cement seem to
be costly solutions. Does anyone have any experience with this sort of
thing? Can the slab be reinforced to hold the cracks tight? How much
reinforcing would be required?